1. Field of The Invention
The present invention relates to alarm systems in general and, in particular to supervision circuits in a signalling, warning, or alarm system that extend between a sensor and a signalling control unit, warning control unit, or alarm control unit.
2. Background Information
Supervision circuits in such systems generally provide some capability to detect a circuit fault, and/or an attempt to defeat the circuit by manipulation of the wiring. Three forms of supervision are typically provided and will now be briefly summarized with reference to FIGS. 1A to 1D.
In a first form of supervision shown in FIG. 1A, a voltage is generated by a control unit and routed through normally closed switch contacts inside a sensor. Should the sensor be activated, i.e., "alarm" the normally closed switch contacts open, signalling an alarm condition to the control unit by a change in the current flowing therethrough. Should the supervision wiring be cut (opened), accidentally or intentionally, the control unit will also detect a change in the current flowing in the supervision circuit and signal an alarm.
This first method of supervision, however, provides no protection against compromise by someone shorting the supervision circuit wiring and effectively bypassing the sensor and cannot distinguish between a fault, "an alarm," and an intentional tampering condition.
In a second form of supervision shown in FIGS. 1B and 1C, a voltage is generated by a control unit and a current routed through a resistor located at the sensor in series or in parallel with the sensor contacts. For normally closed type sensor contacts as in FIG. 1B, this resistor is placed in series with the contacts. For normally open type sensor contacts as in FIG. 1C, the resistor is placed in parallel with the contacts. The control unit monitors the voltage and current in the supervision circuit. Activation of the sensor contacts either opens (FIG. 1B) or shorts (FIG. 1C) the supervision circuit, and this is detected at the control unit which signals an alarm. Should the supervision wiring between the control unit and the sensor be cut (opened) or jumpered (shorted) accidentally or intentionally, the control unit will detect a change in voltage and/or current and signal an alarm.
In a third form of supervision as shown in FIG. 1D, a voltage is generated by a control unit and a current routed through two resistors located at the sensor. One resistor is placed in series with the sensor contacts, and the other resistor is placed in parallel with them. Thus, regardless of the state of the sensor contacts, i.e., opened or closed, the voltage seen at the control unit will be greater than the lowest possible level, e.g., in a shorted condition, and less than the greatest possible value, e.g., in an open condition. Should the supervision wiring be cut (opened) or jumpered (shorted), accidentally or intentionally, the control unit will see the voltage change to its highest level or lowest level, respectively. In this way, the control unit can distinguish between alarm conditions generated by the sensor and trouble conditions with the supervision circuit.
This third method of supervision provides protection against compromise by jumpering or cutting, but it can still be defeated with relatively simple equipment and knowledge of the circuit. Insertion into the circuit of a regulated voltage source, for example, which can provide an appropriate voltage to the control unit, allows for removing and defeating the sensor circuit after attachment of the voltage source. Once the sensor has been removed, the protected area is of course vulnerable. A need has existed for a supervision method and/or circuitry which overcomes this vulnerability in existing systems.
The present invention was developed to overcome the limitations and thereby protect the existing circuits against such a threat without necessitating removal of existing resistive sensor devices which might already be installed, and which it might not be desirable to replace in the process of enhancing the supervision circuitry.